1. Field of the Invention
The present invention relates to a filter circuit having a transconductor and a capacitor, and more specifically to a transconductor for use in such a filter circuit.
2. Description of the Related Art
A transconductor can provide an output current which is proportional to an input voltage wherein the transconductance is a proportional constant. Therefore, the transconductor is used in a filter circuit, an amplifier, a current transformer, a calibrator, etc.
In a disk system such as a DVD system, in order to ensure compatibility with various media and multiple speed reproduction, a filter circuit used for a signal processing with a disk needs to process signals in a wide speed range from a high speed signal to a low speed signal. The high speed signal is about 100 times higher than the low speed signal. Therefore, the maximum value of a cutoff frequency Fc should be about 100 times or more greater than the minimum value of the cutoff frequency Fc. FIG. 11 shows an example of a transconductor used in such a high speed filter circuit. The high-speed filter circuit of FIG. 11 is an example of a conventional GM-C filter circuit including a transconductor (GM circuit) and capacitors (C). Specifically, in the GM-C filter circuit shown in FIG. 11, a plurality of capacitors 902 which are in parallel to one another are connected through a plurality of switches 903 to a transconductor (GM circuit) 901. The cutoff frequency (Fc), which is a filter characteristic of the GM-C filter circuit, is represented by expression (1) of gm (transconductance of the transconductor) and C (capacitance):
Fc=gm/C xe2x80x83xe2x80x83(1) 
Thus, in order to improve the filter characteristic, it is necessary to extend the variation range of the cutoff frequency Fc. For that purpose, as seen from expression (1), it is necessary to extend the variation range of transconductance gm or the variation range of capacitance C.
According to the conventional technology, it is very difficult to extend the variation range of the transconductance gm of the transconductor 901. In particular, it is more difficult to extend the variation range of transconductance in a transconductor formed by a CMOS transistor than in a transconductor formed by a bipolar transistor. The transconductor formed by a CMOS transistor typically has a transconductance which varies over a variation range wherein the maximum limit is only about 10 times greater at most than the minimum limit. Therefore, in the case of making a filter circuit shown in FIG. 11 using a CMOS transistor with such a narrow variation range of conductance, capacitors 902 to be connected to the transconductor 901 are selected by the switches 903 to change the entire capacitance C, whereby the cutoff frequency is changed. That is, by making the capacitance C variable as well as the transconductance gm of the transconductor 901, the variation range of the cutoff frequency is extended.
Now, consider a case where the cutoff frequency needs to have a variation range wherein the maximum limit is about 100 times greater than the minimum limit, while the transconductance gm of the transconductor 901 has a variation range wherein the maximum limit is only about 10 times greater than the minimum limit. In this case, the whole capacitance C of the capacitors 902 needs to have a variation range in which the maximum limit is about 10 times greater than the minimum limit by the selection of the switches 903. The minimum value of the whole capacitance C of the capacitors 902 is limited due to noise, stability of circuit performance, difference in capacitance among the respective capacitors, etc. Accordingly, the capacitance C of the capacitors 902 cannot be significantly reduced. Thus, in the case where a filter circuit includes a plurality of capacitors 902 so that the total capacitance in the filter circuit is changed by various combinations of the capacitors 902 over a variation range wherein the maximum limit is about 10 times greater than the minimum limit, the circuit area increases. Furthermore, the on-resistance of the switch 903 which is directly connected to the capacitor 902 deteriorates the group delay of the filter circuit.
Thus, it is preferable to provide a transconductor which has a variation range of transconductance wherein the maximum limit is about 100 times or more greater than the minimum limit. With a transconductor having such a wide variation range of transconductance, a faster filter circuit, a faster current transformer, etc., can be achieved.
According to one aspect of the present invention, a transconductor has a transconductance gm and receives an input voltage Vin and outputs in response to the input voltage Vin an output current Iout of gmxc3x97Vin , wherein: the transconductor includes a plurality of sub-transconductors which are connected in parallel to one another: and at least one control signal is input to the plurality of sub-transconductors, and the plurality of sub-transconductors are controlled by the at least one control signal such that at least one of the plurality of sub-transconductors has a negative transconductance, whereby the transconductance gm of the transconductor can be varied.
In one embodiment of the present invention, the at least one of sub-transconductors includes a differential input/output transconductor and a plurality of switching sections, the plurality of switching sections are connected to a first input terminal and a second input terminal of the differential input/output transconductor, and the plurality of switching sections are switched in response to the at least one control signal, thereby switching a sign of a transconductance of the differential input/output transconductor.
In another embodiment of the present invention, the plurality of sub-transconductors include one or more first sub-transconductors having a first polarity transconductance and one or more second sub-transconductors having a second polarity transconductance; and an operation (on/off) state of each of the one or more first and second sub-transconductors is selectively switched by the at least one control signal.
In still another embodiment of the present invention, each of the one or more first and second sub-transconductors is a differential input/output transconductor.
In still another embodiment of the present invention, the differential input/output transconductor has an input terminal with a first polarity, an input terminal with a second polarity, an output terminal with a fist polarity, and an output terminal with a second polarity; a difference between a first input voltage which is input to the input terminal with a first polarity and a second input voltage which is input to the input terminal with a second polarity is equal to the input voltage; and a difference between a first output current which is output from the output terminal with a first polarity and a second output current which is output from the output terminal with a second polarity is equal to the output current.
In still another embodiment of the present invention, the transconductor further includes: a first transistor with a first polarity which has a source connected to a power supply with a first polarity, a gate connected to a bias terminal, and a drain connected to the output terminal with a fist polarity; a second transistor with a first polarity which has a source connected to the power supply with a fist polarity, a gate connected to the bias terminal, and a drain connected to the output terminal with a second polarity; first and second sub-transconductor components each having at least one unit transconductor, the first and second sub-transconductor components being connected to the output terminal with a first polarity; and third and fourth sub-transconductor or components each having at least one unit transconductor, the third and fourth sub-transconductor components being connected to the output terminal with a second polarity, wherein the first and third sub-transconductor components are connected to the first input terminal to which the first input voltage is input, and the second and fourth sub-transconductor components are connected to the second input terminal to which the second input voltage is input, whereby a sub-transconductor formed by the first and fourth sub-transconductor components has a transconductance with a first polarity, and a sub-transconductor formed by the second and third sub-transconductor components has a transconductance with a second polarity, and by the at least one control signal which is input to the first and fourth sub-transconductor components and to the second and third sub-transconductor components, the transconductance with a first polarity of sub-transconductor formed by the first and fourth sub-transconductor components and the transconductance with a second polarity of sub-transconductor formed by the second and third sub-transconductor components are controlled.
In still another embodiment of the present invention, the control signal which is input to the first and fourth sub-transconductor components and the control signal which is input to the second and third sub-transconductor components are different.
In still another embodiment of the present invention, the unit transconductor includes: a third transistor with a second polarity which has a gate to which one of the first and second input voltages is input and a source connected to a power supply with a second polarity, and a fourth transistor with a second polarity which has a gate to which the control signal is input, a source connected to a drain of the third transistor, and a drain connected to one of the output terminal with a fist polarity and the output terminal with a second polarity.
In still another embodiment of the present invention, each of the first to fourth sub-transconductor components includes a pair of unit transconductors.
In still another embodiment of the present invention, the sub-transconductor component includes: a third transistor with a second polarity which has a gate to which one of the first and second input voltages is input and a source connected to a power supply with a second polarity; a fourth transistor with a second polarity which has a gate to which the control signal is input, a source connected to a drain of the third transistor, and a drain connected to one of the output terminal with a first polarity and the output terminal with a second polarity; a fifth transistor with a second polarity which has a gate to which one of the first and second input voltages is input and a source connected to a power supply with a second polarity; and a sixth transistor with a second polarity which has a gate to which the control signal is input, a source connected to a drain of the fifth transistor, and a drain connected to the output terminal to which a drain of the fourth transistor is connected.
In still another embodiment of the present invention, the control signal which is input to the fourth transistor and the control signal which is input to the sixth transistor are different.
According to another aspect of the present invention, a filter circuit includes: the transconductor of claim 1; and a capacitor connected to the transconductor.
In one embodiment of the present invention, a plurality of the filter circuits are connected into a ladder arrangement or a cascade arrangement.
According to still another aspect of the present invention, a filter circuit includes: a plurality of transconductors, each of the plurality of transconductors outputting an output current which is proportional to an input voltage, each of the plurality of transconductors having an input terminal to which the input voltage is input and an output terminal from which the output current is output; a plurality of capacitors; a plurality of first switching sections; and a plurality of second switching sections, wherein each of the plurality of capacitors is connected to an output terminal of at least one of the plurality of transconductors, each of the plurality of first switching sections is connected to an input terminal of one of the plurality of transconductors, each of the plurality of second switching sections is connected to an output terminal of one of the plurality of transconductors, and a transconductor to be selected among the plurality of transconductors can be controlled by a plurality of first control signals which are input to each of the plurality of transconductors and a second control signal which is input to each of the plurality of first and second switching sections.
In one embodiment of the present invention, the plurality of transconductors and the plurality of capacitors are connected into a ladder arrangement or a cascade arrangement.
In another embodiment of the present invention, the filter circuit further includes a measurement section for measuring a transconductance of the transconductor, wherein the transconductor to be selected among the plurality of transconductors is turned on by the first control signal, and in response to the turning on, the first switching section connected to an input terminal of the selected transconductor and the second switching section connected to an output terminal of the selected transconductor are turned on by the second control signal, whereby the measurement section measures a transconductance of the transconductor.
In still another embodiment of the present invention, the first control signal finely adjusts the measured transconductance of the transconductor based on the measured transconductance of the transconductor.
In still another embodiment of the present invention, each of the plurality of transconductors which are connected in parallel to one another; and a control signal is input to the plurality of sub-transconductors, and the plurality of sub-transconductors are controlled by the control signal such that at least one of the plurality of sub-transconductors has a negative transconductance, whereby the transconductance of the transconductor can be varied.
Thus, the invention described herein makes possible the advantages of (1) providing a transconductor having a wide variation range of transconductance; and (2) providing a filter circuit in which a transconductor having a wide variation range of transconductance is used, whereby filter characteristics such as cutoff frequency, the Q-factor (quality factor), etc., can be varied without employing a large capacitor which would increase the device area.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.